1. Field of the Invention
The present invention relates to an electrophotographic apparatus using a charging apparatus for mainly charging the surface of a photosensitive material by contact charging.
2. Description of the Prior Art
In an image-forming method based upon the electrophotographic method, an image is formed by uniformly charging the surface of a photosensitive material, exposing the surface of the photosensitive material to the image-bearing light to form on the surface of the photosensitive material an electrostatic latent image that corresponds to the image of the document, and developing and transferring the electrostatic latent image.
In such an image-forming method, the surface of the photosensitive material is usually charged (mainly charged) by the corona charging accompanied, however, by the generation of ozone which contaminates the environment. In order to avoid the generation of ozone, a method has recently been proposed to mainly charge the surface of the photosensitive material by bringing an electrically conducting rubber roller into frictional contact with the surface of the photosensitive material while applying a bias voltage (Japanese Laid-Open Patent Publications Nos. 149669/1988 and 267667/1989).
According to the above-mentioned charging method based upon the frictional contact, however, the presence of foreign matters such as dust, paper powder, etc. between the electrically conducting rubber roller and the photosensitive material impairs the uniformity in the charging, and makes it difficult to stably carry out the charging. In forming the image, furthermore, when the surface of the photosensitive material is not so clean permitting the toner to stay thereon, then the residual toner may stick to the surface of the photosensitive material to deteriorate the durability of the photosensitive material. To carry out the uniform charging, furthermore, application of the DC bias voltage only is not sufficient and an AC bias voltage must be applied in combination.
In order to solve the above-mentioned problems, the present applicant has previously proposed a method of effecting the frictional charging by bringing a flexible and electrically conducting sheet into frictional contact with the surface of the photosensitive material by using a charging apparatus equipped with an electrically conducting brush roller while applying a DC voltage to the above roller (see Japanese Patent Application No. 68148/1992). The charging apparatus used for the above charging method usually comprises an electrically conducting brush roller and a flexible and hollow electrically conducting roller which contains the above brush roller and is so provided as to come into contact with the brush.
The above charging apparatus is very excellent in that the flexible and electrically conducting sheet constituting a hollow and electrically conducting roller is brought into intimate contact with the surface of the photosensitive material being pushed by the electrically conducting brush and, hence, frictional charging is uniformly carried out by simply applying a low DC bias voltage only to the electrically conducting brush without the need of applying an AC bias voltage.
However, the flexible and electrically conducting sheet that constitutes the hollow and electrically conducting roller rotates in contact with the surface of the photosensitive material being simply pushed by the Force of the brush. That is, the electrically conducting sheet rotates unstably giving rise to the occurrence of irregular rotation, slipping relative to the photosensitive material, and friction with respect to the surface of the photosensitive material. As a result, the photosensitive material is worn out and loses durability.
Furthermore, the electrically conducting sheet is usually very thin and may be twisted by the rotational driving force of the brush roller, resulting in the development of irregular charging. Moreover, the electrically conducting sheet may be broken as it is twisted constantly and more greatly.
According to the above charging apparatus in which the hollow and electrically conducting roller is press-contacted to the surface of the photosensitive material by the electrically conducting brush that is contained therein, hairs of the electrically conducting brush remain fallen down permanently when the copying operation is not carried out for extended periods of time. Consequently, the hollow and electrically conducting roller is deformed and makes it difficult to carry out the uniform charging. Moreover, foreign matters such as toner and dust adhered onto the surface of the photosensitive material migrate onto the surface of the hollow and electrically conducting roller and build up thereon. After used for extended periods of time, therefore, troubles develop such as irregular charging giving damage to the surface of the photosensitive material, which is a problem from the standpoint of life of the apparatus.